1. Field of the Invention
The present invention relates to an after-air nozzle for a two-stage combustion boiler, and to a structure of a two-stage combustion boiler using the after-air nozzle.
2. Prior Art
Boilers are required to reduce the concentrations of nitrogen oxides (NOx), and a two-stage combustion method is applied to meet this requirement. In this method, a fuel is burnt under a state of air shortage and then air for complete combustion is supplied from an after-air nozzle. For the after-air nozzle, several structures are proposed for improved fuel-air mixture and combustion states.
For example, as illustrated in FIG. 1 of Patent Reference 1 (Japanese Laid-Open Patent Application Publication No. Hei 10-122546), a structure is proposed that has a vena contracta with its air flow passage outside diameter progressively diminished towards an air-jetting port, in an after-air nozzle. However, although recent boilers are required to reduce NOx and carbon oxide (CO) concentrations at the same time, conventional constructions have only been able to reduce either NOx or CO emissions.
Patent Reference 1: Japanese Laid-Open Patent Application Publication No. Hei 10-122546